//! CUBELinux-2 DBT (dynamic binary translation) runtime.
//!
//! Per the PDF (Package 4, §549): *"cubedbt: a runtime that reads CZYX‑stored
//! translation rules and code fragments and patches them into a code cache to
//! execute mimicked behavior."*
//!
//! This crate is the substrate for the PDF's "watch a binary, archive its
//! behavior structurally, replay or transform it as if it were its own program"
//! story (§554). It is deliberately dependency-free and operates on the real
//! `CubeStore` / `CodeCell` / `Vm` types from the rest of the workspace, so it
//! is exercisable today on `HashBackend` and slots into `ConcurrentStore`
//! later without an API change.
//!
//! Model
//! -----
//! * A **translation rule** is a CZYX record (`Kind::Variant`) whose body is a
//!   serialized [`TranslationRule`]. It names an *op kind* it can replace and
//!   carries a *replacement fragment* (a `Vec<Op>`). The rule record links to
//!   the original code cell it may substitute (the association edge).
//! * A **code cache** is the patched working set: `patch()` rewrites an
//!   original `CodeCell`'s bytecode under the eligible rules and writes the
//!   result back as a new record (preserving the original header's kind/title
//!   and behavior descriptors, so it runs through the same `Vm`).
//! * A **DBT runtime** discovers rules in a store and can `mimic` a target
//!   coordinate: fetch original → apply rules → run the patched version in the
//!   `Vm`, returning the `RunResult`.

use cubecode::{opcode::Op, CodeCell, Kind, Vm};
use cubecoords::{CubeHeader, Czyx};
use cubestore::{CubeBackend, CubeStore, HashBackend};

/// `C` axis band where DBT translation rules are stored, kept distinct from the
/// `c210` OS-kernel band (`cubecode::C_OS_KERNEL`) and the `c200` snapshot data.
pub const C_DBT_RULE: u8 = 220;

/// `C` axis band where patched/mimicked code-cache entries are written.
pub const C_DBT_CACHE: u8 = 221;

/// The `Op` kinds a translation rule can target.
#[derive(Copy, Clone, Debug, PartialEq, Eq)]
pub enum OpClass {
    Const,
    Add,
    Sub,
    Mul,
    Div,
    Mod,
    And,
    Or,
    Xor,
    Shl,
    Shr,
    Eq,
    Ne,
    Lt,
    Gt,
    Le,
    Ge,
    Load,
    Store,
    CallLink,
    Any,
}

impl OpClass {
    /// Classify a live [`Op`].
    pub fn of(op: &Op) -> OpClass {
        match op {
            Op::Const(_) => OpClass::Const,
            Op::Add => OpClass::Add,
            Op::Sub => OpClass::Sub,
            Op::Mul => OpClass::Mul,
            Op::Div => OpClass::Div,
            Op::Mod => OpClass::Mod,
            Op::And => OpClass::And,
            Op::Or => OpClass::Or,
            Op::Xor => OpClass::Xor,
            Op::Shl => OpClass::Shl,
            Op::Shr => OpClass::Shr,
            Op::Eq => OpClass::Eq,
            Op::Ne => OpClass::Ne,
            Op::Lt => OpClass::Lt,
            Op::Gt => OpClass::Gt,
            Op::Le => OpClass::Le,
            Op::Ge => OpClass::Ge,
            Op::Load(_) => OpClass::Load,
            Op::Store(_) => OpClass::Store,
            Op::CallLink(_) => OpClass::CallLink,
            _ => OpClass::Any,
        }
    }
}

/// A translation rule: a named, CZYX-addressed mapping from an op class to a
/// replacement bytecode fragment. Serialized into a rule record's body.
#[derive(Clone, Debug, PartialEq, Eq)]
pub struct TranslationRule {
    /// Human name (also the rule record's title).
    pub name: String,
    /// Which op class this rule can substitute.
    pub target: OpClass,
    /// Replacement bytecode fragment (must itself be valid; it replaces every
    /// matched op in the original, in place, preserving program length-agnostic
    /// semantics the caller is responsible for).
    pub fragment: Vec<Op>,
}

impl TranslationRule {
    /// Serialize to a stable byte form (name length-prefixed, target byte,
    /// fragment as opcode codec). No external deps.
    pub fn to_bytes(&self) -> Vec<u8> {
        let mut out = Vec::new();
        let nb = self.name.as_bytes();
        out.push(nb.len() as u8);
        out.extend_from_slice(nb);
        out.push(self.target as u8);
        out.push(self.fragment.len() as u8);
        for op in &self.fragment {
            out.extend_from_slice(&cubecode::opcode::encode(&[*op]));
        }
        out
    }

    /// Inverse of [`to_bytes`]. Returns `None` on any malformed input.
    pub fn from_bytes(b: &[u8]) -> Option<TranslationRule> {
        let mut i = 0;
        let nlen = *b.get(i)? as usize;
        i += 1;
        if i + nlen > b.len() {
            return None;
        }
        let name = String::from_utf8(b[i..i + nlen].to_vec()).ok()?;
        i += nlen;
        let target = *b.get(i)?;
        i += 1;
        let target = match target {
            0 => OpClass::Const,
            1 => OpClass::Add,
            2 => OpClass::Sub,
            3 => OpClass::Mul,
            4 => OpClass::Div,
            5 => OpClass::Mod,
            6 => OpClass::And,
            7 => OpClass::Or,
            8 => OpClass::Xor,
            9 => OpClass::Shl,
            10 => OpClass::Shr,
            11 => OpClass::Eq,
            12 => OpClass::Ne,
            13 => OpClass::Lt,
            14 => OpClass::Gt,
            15 => OpClass::Le,
            16 => OpClass::Ge,
            17 => OpClass::Load,
            18 => OpClass::Store,
            19 => OpClass::CallLink,
            _ => OpClass::Any,
        };
        let flen = *b.get(i)? as usize;
        i += 1;
        let mut fragment = Vec::new();
        for _ in 0..flen {
            // Each fragment op was encoded individually; decode one at a time,
            // advancing `i` by its encoded byte length.
            let one = decode_one(&b[i..])?;
            i += one.len;
            fragment.push(one.op);
        }
        Some(TranslationRule {
            name,
            target,
            fragment,
        })
    }
}

/// One decoded opcode plus its encoded length, used by `from_bytes`.
struct One {
    op: Op,
    len: usize,
}

fn decode_one(b: &[u8]) -> Option<One> {
    let op = cubecode::opcode::decode(b).ok()?;
    let op = op.into_iter().next()?;
    let len = cubecode::opcode::encode(&[op]).len();
    Some(One { op, len })
}

/// A patched working set ("code cache") inside a [`CubeStore`].
///
/// `patch` rewrites an original [`CodeCell`]'s bytecode under the eligible
/// rules and writes the result back as a new record (preserving the original
/// header's kind/title and behavior descriptors, so it runs through the same
/// `Vm`). This is the literal "patches them into a code cache to execute
/// mimicked behavior" step from the PDF §549.
pub struct CodeCache {
    store: CubeStore<HashBackend>,
    /// Coordinate of the next cache slot to allocate.
    next_x: u8,
}

impl CodeCache {
    /// Build an empty cache over a fresh in-memory backend.
    pub fn new() -> Self {
        CodeCache {
            store: CubeStore::new(HashBackend::new()),
            next_x: 1,
        }
    }

    /// Patch `original` under `rules`: every op whose class matches a rule's
    /// target (or whose rule target is `Any`) is replaced by that rule's
    /// fragment. The rewritten cell is stored at a fresh cache coordinate and
    /// returned (with its new label). The original store is untouched.
    pub fn patch(&mut self, original: &CodeCell, rules: &[TranslationRule]) -> CodeCell {
        let mut patched: Vec<Op> = Vec::with_capacity(original.code.len());
        for op in &original.code {
            let class = OpClass::of(op);
            let mut applied = false;
            for r in rules {
                if r.target == class || r.target == OpClass::Any {
                    patched.extend_from_slice(&r.fragment);
                    applied = true;
                    break;
                }
            }
            if !applied {
                patched.push(*op);
            }
        }
        let label = Czyx::new(C_DBT_CACHE, 1, 1, self.next_x);
        self.next_x = self.next_x.wrapping_add(1).max(1);
        let (h, body) = cache_header_for(original, &cubecode::opcode::encode(&patched));
        self.store.put_record(label, &h, &body);
        CodeCell::from_record(label, &h, &body).expect("patched cell is always valid bytecode")
    }

    /// Run a patched cache entry in the VM, returning its result.
    pub fn run(&self, cell: &CodeCell) -> cubecode::RunResult {
        let mut vm = Vm::new(self.store.clone());
        vm.run(cell.label)
    }

    /// Borrow the backing store (e.g. to persist or inspect).
    pub fn store(&self) -> &CubeStore<HashBackend> {
        &self.store
    }
}

impl Default for CodeCache {
    fn default() -> Self {
        Self::new()
    }
}

/// Build a cache record header that preserves the original's kind/title and
/// behavior descriptors, but marks it a `Variant` (a mimicked implementation).
fn cache_header_for(original: &CodeCell, body: &[u8]) -> (CubeHeader, Vec<u8>) {
    let mut h = CubeHeader::new();
    h.title = original.name().map(|s| format!("mimic:{}", s));
    h.doc_type = Some(Kind::Variant.as_str().into());
    h.size_bytes = Some(body.len() as u64);
    // Carry the original's behavior descriptors forward (spec: descriptors
    // travel with the mimicked behavior).
    h.flags.0 |= original.header.flags.0 & cubecode::HEADER_FLAG_BEHAVIOR;
    h.refresh_flags();
    (h, body.to_vec())
}

/// The DBT runtime: discovers CZYX-stored translation rules and can `mimic` a
/// target code cell by patching it under those rules and running the result.
pub struct DbRuntime<B: CubeBackend> {
    store: CubeStore<B>,
}

impl<B: CubeBackend> DbRuntime<B> {
    /// Wrap a store that already contains rule records (or will, via
    /// [`store_rule`]). The runtime is read-only over this store.
    pub fn new(store: CubeStore<B>) -> Self {
        DbRuntime { store }
    }

    /// Collect every translation rule currently in the `c220` rule band.
    pub fn discover_rules(&self) -> Vec<TranslationRule> {
        let mut rules = Vec::new();
        for k in self.store.keys() {
            if k.c != C_DBT_RULE {
                continue;
            }
            if let Some((_, body)) = self.store.get_record(&k) {
                if let Some(r) = TranslationRule::from_bytes(&body) {
                    rules.push(r);
                }
            }
        }
        rules
    }

    /// Fetch a code cell by coordinate from the wrapped store.
    pub fn fetch(&self, label: Czyx) -> Option<CodeCell> {
        let (h, b) = self.store.get_record(&label)?;
        CodeCell::from_record(label, &h, &b)
    }

    /// Mimic `target`: fetch it, apply all discovered rules, patch into a fresh
    /// cache, and run the patched version — returning the `RunResult`. This is
    /// the end-to-end "re-run or modify behavior without the original binary"
    /// path (PDF §554).
    pub fn mimic(&self, target: Czyx) -> Option<cubecode::RunResult> {
        let original = self.fetch(target)?;
        let rules = self.discover_rules();
        let mut cache = CodeCache::new();
        let patched = cache.patch(&original, &rules);
        Some(cache.run(&patched))
    }
}

/// Store a translation rule into the `c220` rule band at a fresh coordinate.
/// Returns the coordinate it was written to.
pub fn store_rule<B: CubeBackend>(store: &mut CubeStore<B>, rule: &TranslationRule, x: u8) -> Czyx {
    let label = Czyx::new(C_DBT_RULE, 1, 1, x);
    let mut h = CubeHeader::new();
    h.title = Some(rule.name.clone());
    h.doc_type = Some(Kind::Variant.as_str().into());
    h.size_bytes = Some(rule.to_bytes().len() as u64);
    h.refresh_flags();
    store.put_record(label, &h, &rule.to_bytes());
    label
}

#[cfg(test)]
mod tests {
    use super::*;
    use cubecode::opcode::Op;
    use cubecode::RunResult;
    use cubecoords::Czyx;

    fn sample_cell(coord: Czyx, code: Vec<Op>) -> CodeCell {
        let mut h = CubeHeader::new();
        h.title = Some("sample".into());
        h.doc_type = Some(Kind::Fn.as_str().into());
        h.refresh_flags();
        CodeCell::from_record(coord, &h, &cubecode::opcode::encode(&code))
            .expect("sample is valid bytecode")
    }

    #[test]
    fn rule_round_trips_through_bytes() {
        let r = TranslationRule {
            name: "double-add".into(),
            target: OpClass::Add,
            fragment: vec![Op::Const(2), Op::Mul],
        };
        let bytes = r.to_bytes();
        let back = TranslationRule::from_bytes(&bytes).expect("rule decodes");
        assert_eq!(r, back);
    }

    #[test]
    fn cache_patches_and_runs() {
        // Original: Const 3, Const 4, Add, Halt => 3+4 = 7.
        let orig = sample_cell(
            Czyx::new(1, 1, 1, 1),
            vec![Op::Const(3), Op::Const(4), Op::Add, Op::Halt],
        );
        // Rule: replace `Add` with `Const 2, Mul` => (a)*(2). With a=3,b=4:
        // naive in-place substitution yields Const3, Const4, Const2, Mul =>
        // 4*2 = 8. This proves the patched fragment is what runs.
        let rule = TranslationRule {
            name: "mul-by-2".into(),
            target: OpClass::Add,
            fragment: vec![Op::Const(2), Op::Mul],
        };
        let mut cache = CodeCache::new();
        let patched = cache.patch(&orig, &[rule]);
        assert_eq!(patched.kind(), Kind::Variant, "cache entry is a Variant");
        match cache.run(&patched) {
            RunResult::Halted { top: Some(v) } => assert_eq!(v, 8, "patched semantics differ"),
            other => panic!("patched run failed: {:?}", other),
        }
    }

    #[test]
    fn runtime_mimic_end_to_end() {
        let mut store = CubeStore::new(HashBackend::new());
        // Original target at c1.
        let target = Czyx::new(1, 1, 1, 1);
        let orig = sample_cell(target, vec![Op::Const(10), Op::Const(5), Op::Sub, Op::Halt]);
        store.put_record(orig.label, &orig.header, &orig.body());

        // Rule in the c220 band: replace Sub with Add (10+5=15 instead of 5).
        let rule = TranslationRule {
            name: "sub->add".into(),
            target: OpClass::Sub,
            fragment: vec![Op::Add],
        };
        store_rule(&mut store, &rule, 1);

        let rt = DbRuntime::new(store);
        let rules = rt.discover_rules();
        assert_eq!(rules.len(), 1, "rule discovered in c220 band");
        match rt.mimic(target) {
            Some(RunResult::Halted { top: Some(v) }) => assert_eq!(v, 15, "mimic applied rule"),
            other => panic!("mimic failed: {:?}", other),
        }
    }

    #[test]
    fn mimic_without_rules_matches_original() {
        let mut store = CubeStore::new(HashBackend::new());
        let target = Czyx::new(1, 1, 1, 2);
        let orig = sample_cell(target, vec![Op::Const(7), Op::Const(2), Op::Mul, Op::Halt]);
        store.put_record(orig.label, &orig.header, &orig.body());
        let rt = DbRuntime::new(store);
        match rt.mimic(target) {
            Some(RunResult::Halted { top: Some(v) }) => assert_eq!(v, 14),
            other => panic!("mimic without rules should run original: {:?}", other),
        }
    }
}